What makes interesting clouds happens in the troposphere extending about 15km above the equator, 10kms over the poles (9.5 and 6.3 miles). And here, too, lives the most successful form of life – bacteria. Clouds form when microscopic specs of dust or dirt act as nucleators around which water vapor freezes. They clump together and form clouds. Up there it needs to be – 35 degrees C for freezing to start (-31F). But around organic nuclei, like bacteria, the freezing starts much earlier. Thus, bacteria facilitate cloud formation.A new field of research deals with the effects bacteria might have on climate and our daily weatherhttp://www.wired.com/science/planetearth/news/2008/02/bacteria_clouds﻿